There are two types of bearing apparatus of a vehicle, one for a driving wheel and one for a driven wheel. Especially in the vehicle bearing apparatus that rotatably supports a wheel relative to a suspension apparatus, it is required not only that it be made at a low cost but additionally it must be light weight and have a small size to improve fuel consumption. A representative example of a prior art bearing apparatus for a vehicle driving wheel is shown in FIG. 4.
FIG. 4 illustrates a vehicle wheel bearing apparatus of third generation type having an inner member 53 with a wheel hub 51 and an inner ring 52. An outer member 54 is press fit onto the inner member 53. Double row rolling elements 55 and 55 are arranged between the outer and inner members 54 and 53. The wheel hub 51 is integrally formed at one end with a wheel mounting flange 56 to mount a vehicle wheel (not shown). Also, the wheel hub 51 has an inner raceway surface 51a on its outer circumferential surface. A cylindrical portion 51b axially extends from the inner raceway surface 51a. A serration 51c is formed on its inner circumferential surface. Hub bolts 56a, for securing the wheel, are arranged via an equidistant space along the circumference of the wheel mounting flange 56. The inner ring 52, including an inner raceway surface 52a formed on its outer surface, is press fit onto the cylindrical portion 51b of the wheel hub 51.
The outer member 54 is integrally formed with a body mounting flange 54b. The outer member 54 also has double row outer raceway surfaces 54a and 54a on its inner circumferential surface. Double row rolling elements 55 and 55 are freely rollably arranged between the double row outer raceway surfaces 54a and 54a and the opposed inner raceway surfaces 51 a and 52a. 
An outer joint member 58, forming a part of a constant velocity universal joint 57, is integrally formed with a stem portion 60 that axially extends from the shoulder portion 59 of the outer joint member 58. A serration 60a is on the stem outer circumferential surface to engage the serration 51c of the wheel hub 51. An outer thread 60b is on its tip end. The outer joint member 58 is fit into the wheel hub 51 until its shoulder portion 59 abuts the inner ring 52. Accordingly, the outer joint member 58 and wheel hub 51 are separably united by fastening a nut 61 on the outer thread 60b. 
In such a vehicle wheel bearing apparatus, the bearing is usually set at a negative gap in order to increase the rigidity of the bearing apparatus and to improve the durability of the bearing portion. In this case the bearing pre-load is controlled by setting the fastening torque of the nut 61 at a predetermined value. However, since the inner ring 52 is mounted on the wheel hub 51 simply by press fitting, the inner ring 52 is liable to be axially displaced on the wheel hub 51 by external forces such as vibration or rotation during transportation and/or assembling operation before the bearing apparatus is assembled to the constant velocity universal joint. Thus, the initially set pre-load is lost and accordingly rattling would occur within the bearing.
If the inner ring 52 is displaced and rattling occurs in the bearing, a lip 63a of an inboard side seal 63 is dropped off from the sliding surface (outer circumferential surface of the inner ring 52) as shown in FIG. 5(b). Under such a condition, if the nut 61 is fastened with a predetermined fastening torque during assembly of the constant velocity universal joint, although the inner ring 52 is returned to its original position, the lip 63a is overturned or pinched between the inner raceway surface 52a and the rolling element 55 and is thus damaged or severed. (FIG. 5(c)). This extremely reduces the sealability of the lip 63a and reduces the life of the bearing due to ingress of muddy water into the bearing which causes seizure of the bearing due to the outflow of lubricating grease from the bearing.
A vehicle wheel bearing apparatus that can solve such a problem of preventing the displacement of the inner ring has been developed as shown in FIG. 6. According to the bearing apparatus of FIG. 6, the axial displacement of an inner ring 62 or its dropping off from a wheel hub 51′ is prevented by a caulked portion 51d formed by plastically deforming an end of a cylindrical portion 51b of the wheel hub 51′. This kind of structure is called a “self-retaining type” and has a feature that can prevent the loss of the pre-load for a long term. Structural elements of the bearing apparatus of FIG. 6 are the same as those of the bearing apparatus of FIG. 4 and are designated by same reference numerals. Repeating of their description will thus be omitted.
In addition, in the vehicle wheel bearing apparatus of FIG. 6, an annular recess 62a is formed at the inboard side end face of the inner ring 62 within which the caulked portion 51d is contained. The hardened projecting end of the inner ring 62 abuts a shoulder portion 59 of an outer joint member 58. Accordingly, since the non-hardened caulked portion 51d does not abut the shoulder portion 59 of the outer joint member 58, it is possible to prevent wear of the caulked portion 51d (see Japanese Laid-open Patent Publication No. 164803/1997).
However in such a vehicle wheel bearing apparatus of FIG. 6, it is difficult to form the caulked portion 51d by plastically deforming (swing caulking) the end of the cylindrical portion 51b radially outward because of interference of the projected end of the annular recess 62a of the inner ring 62 against a caulking tool (not shown). In addition, since it is impossible to restrain the radially outer end of the caulked portion 51d by the caulking tool, the caulking operation has to be carried out by plastically deforming the end of the cylindrical portion 51b of the wheel hub 51′ only in the axial direction. Accordingly, a region of the cylindrical portion 51b near the caulked portion 51d is also deformed radially outward. Thus, the inner ring 62 is also expanded radially outward and this causes a large hoop stress in the inner ring 62.
The generation of a large hoop stress in the inner ring 62 exerts an extremely bad influence on the strength and durability of the inner ring 62. In order to deal with this problem, the amount of plastic deformation of a portion of the inner ring 62 is suppressed where the cylindrical portion 51b of the wheel hub 51′ is fitted during the swing caulking operation. However it is necessary to strictly care for the dispersion of hardness of base metal of the wheel hub 51′ or dispersion of the hardened layers, hardened by high frequency induction hardening, in their axial directions at the end of the cylindrical portion 51b. This causes an increase in the manufacturing cost of the bearing apparatus. Thus, a commonly adopted way to reduce the generated hoop stress in the inner ring is to increase the thickness of the inner ring 62. This adds both to weight and cost.